I have a betta sorority, and I love it. That tank is so interesting!
I quarantined the girls for a couple of weeks, each in a 1/2 or 3/4 gallon container. Which I admit got a bit ridiculous, with 7 containers to keep sparkling clean... But during that time I got to know the girls and their individual personalities.
I didn't even try to put the weakest one in the sorority. I felt that she would never make it. She ended up in a 5.5 gallon with some neon tetras, where she is very happy. I did try to put the most aggressive one in, kind of against my better judgement. I pulled her out almost immediately. She was a shark! I gave her to my office mate, who is really enjoying her.
The remaining five are a perfect mix. They get along just fine. Sometimes when I turn the light on, I see that they are all sleeping together in a patch of floating plants. It's very cute.
So, I guess my advise would be get to know the the girls before throwing them all together, because some of them simply aren't suited for the sorority situation. I had already planned for that, bought a couple of extras, and knew what I would do with them if they didn't fit in that particular tank. (The pet store would have taken them back, but I didn't want to cram them back into a little cup.)
So that's how I put my betta sorority together. Hope it was helpful.